Method for operating a geodetic instrument and associated geodetic instrument

Abstract

The inventive concept relates to a method for operating a geodetic instrument, the geodetic instrument comprising a light source for assisting a user in aiming at a target in a scene, wherein the imaging device and the light source share a common optical path within the geodetic instrument, the method comprising: causing the light source to emit light pulses towards the target; causing the imaging device to capture an image of the scene using a sequence of frames, wherein the frames of the sequence of frames include an exposure time during which the imaging device is exposed to light from the scene; synchronizing the emission of the light pulses with the sequence of frames to obtain data from images in which the light pulses are absent; and processing the obtained data to measure the scene.

Description

Technical Field

[0001] The inventive concept described herein generally relates to geodetic instruments. Background Art

[0002] Optical instruments such as geodetic instruments are commonly used to measure the position of an object to obtain information such as horizontal and vertical angles and distances. Newer instruments typically include an electronic imaging device (e.g., a camera) to acquire a digital image of the object.

[0003] Conventional surveying instruments include a telescope system for observing an object, which can then be imaged on a camera behind the telescope system. Further, such an instrument can include a distance measuring unit to measure the distance to the object observed by the telescope system. The viewing angle of the telescope system is typically small (e.g., 1 to 2 degrees), and the user must position the surveying instrument and adjust the optics of the telescope system such that the object to be observed and measured is exactly within the small field of view of the telescope system and preferably on the optical axis of the telescope system, e.g., in order to measure the distance to the object.

[0004] As more functions are added to the surveying instrument, the complexity of the optical setup and electronic control of the instrument increases. Therefore, there is a need to more efficiently implement aiming assistance for geodetic instruments. Summary of the Invention

[0005] It is an object of the present inventive concept to alleviate, mitigate, or eliminate one or more of the above-mentioned deficiencies and drawbacks in the art, either singly or in combination.

[0006] Generally speaking, the present inventive concept is based on the recognition that an imaging device and a light source for assisting a user in aiming at a target in a scene can share a common optical path in a geodetic instrument. This arrangement enables a compact optical setup. As a result, the calibration of the device and the transformation of the direction with respect to the center of the device can be simplified.

[0007] In addition, it has been recognized that a geodetic instrument having an imaging device and a light source sharing a common optical channel may suffer from crosstalk occurring in the common optical channel. Crosstalk is a result of the internal arrangement of features in the geodetic instrument and thus is generally not affected by external conditions such as ambient light changes or light reflected from a target in the scene by the light source. Such crosstalk may reduce the quality of the image data collected by the imaging device, which may interfere with the measurement, make it difficult to identify features in the image, and degrade the user experience. Crosstalk may take any shape and form detected by the imaging device of the geodetic instrument. In addition, crosstalk may be located at any position relative to the pixels of the imaging device. The shape and form of crosstalk may depend on the beam profile of the light source and / or the optics of the geodetic instrument. Preferably, crosstalk does not saturate any pixel of the imaging device with respect to intensity. This can be achieved, for example, by adjusting the exposure time of the imaging device.

[0008] In addition, the solution to these problems should preferably allow the imaging device and the light source to exhibit being unaffected or minimally affected in their respective operations experienced by the user of the geodetic instrument. Accordingly, the present disclosure also aims to improve the geodetic instrument with respect to crosstalk occurring in the common optical channel.

[0009] According to a first aspect of the inventive concept, these and other objects are achieved in whole or at least in part by a method for operating a geodetic instrument including a light source and an imaging device, the light source for assisting a user in aiming at a target in a scene, wherein the imaging device and the light source share a common optical channel within the geodetic instrument. The method includes: causing the light source to emit light pulses towards the target; causing the imaging device to capture an image of the scene using a frame sequence, wherein frames of the frame sequence include an exposure time during which the imaging device is exposed to light from the scene. The method further includes: synchronizing the emission of the light pulses with the frame sequence to obtain data from images in which the light pulses are absent; and processing the obtained data to measure the scene.

[0010] Accordingly, a simpler and more compact optical setup is achieved, where the light source can be operated to assist in aiming at a target in a scene, and the imaging device can be operated to capture an image of the scene without generating crosstalk in the common optical channel shared with the imaging device. By synchronizing the emission of the light pulses with the frame sequence, images in which the light pulses are absent can be captured. The synchronization can be performed with respect to, for example, the exposure time of each frame of the frame sequence. Thus, during the capture of images using the frame sequence, the light source can be repeatedly switched between an operating state and a non-operating state. As will be further discussed in the present disclosure, by adjusting the frequency of this switching and / or the peak power of the light source, the light source can generate a light spot on the target that is similar or identical to the light spot generated when the light source operates in a continuous mode.

[0011] The frame sequence may include a first subset of frames, and the synchronization may include turning off the light source during the exposure time of each frame of the first subset. Thus, the frame sequence will include at least some frames, namely, the first subset of frames, where the light source is off and thus cannot create any crosstalk in the common optical channel. As will be further discussed in this disclosure, the first subset of frames can be utilized in a variety of different ways to partially or fully achieve the above objectives. It should be further understood that different embodiments of the first subset of frames can be advantageously combined.

[0012] According to an embodiment, the frame sequence further includes a second subset of frames, the first subset and the second subset being mutually exclusive subsets, and the synchronization further includes: turning on the light source during at least a portion of each frame of the second subset. The frame sequence may include frames from the first subset that are interleaved with one or more frames from the second subset, or wherein the frame sequence includes frames from the second subset that are interleaved with one or more frames from the first subset. Thus, the frame sequence may include one or more frames in which the light source is on, the one or more frames being interleaved with one or more frames in which the light source is off.

[0013] The method may further include discarding the data associated with each frame of the second subset. Thus, a modified frame sequence is formed that contains only the first subset of frames (i.e., the frames in which the light source is off). At each position of the discarded frames belonging to the second subset of frames, a previous frame belonging to the first subset of frames may be inserted. In other words, the method may further include replacing each frame of the second subset of frames with the corresponding previous frame of the first subset of frames. Thus, the modified frame sequence will include repetitions of one or more frames in the first subset of frames. Depending on the respective numbers of frames in the frame sequence belonging to the first subset and the second subset of frames, the number of repeated frames will vary. Additionally, if such a modified frame sequence with repeated frames is displayed on a display unit, the user will experience a lower frame rate because some frames are displayed for a longer period of time.

[0014] It is also contemplated that the discarded data may leave empty frames in the frame sequence. Such a modified frame sequence will contain frames belonging to the first subset in which the laser is off, as well as empty frames belonging to the second subset. Similarly, if such a modified frame sequence with empty frames is displayed on a display unit, the user will experience a lower frame rate.

[0015] The second subset may include every Nth frame of the frame sequence, where N ranges from 2 to 10. It is also contemplated that the first subset may include every Nth frame of the frame sequence, where N ranges from 2 to 10. It will be understood that when every Nth frame of the frame sequence is a frame of the second subset, the remaining frames of the frame sequence may be frames of the first subset, and vice versa. The occurrence of frames belonging to the first and second subsets of frames (i.e., their frequency in the frame sequence) may be determined based on the frame rate of the imaging device and / or the peak power of the light source. For example, an imaging device with a relatively high frame rate may allow a higher ratio of frames of the second subset to frames of the first subset of the frame sequence, since the frames belonging only to the first subset (i.e., the frames with the light source off) may provide sufficient frame rate to be displayed to the user on the display device. Additionally, the peak power of the light source may be increased to compensate for the turning off of the light source in the frames belonging to the first subset of frames. Since the integration of the human eye is relatively slow, the user may perceive the light emitted from a light source with increased peak power and a duty cycle less than 100% as having the same brightness as the light emitted from a light source with a lower peak power that is on all the time (i.e., a duty cycle of 100%). Preferably, the peak power is selected such that the light source meets safety requirements and remains within the classification in the relevant field of implementation.

[0016] According to another embodiment, the frame further includes at least one intermission time during which the imaging device is configured not to record light from the scene, and the method further includes: for at least some frames of the frame sequence or the first subset, turning on the light source during at least a portion of the at least one intermission time. Thus, the imaging device is configured not to record light from the scene during the intermission time and will therefore not capture any crosstalk in the common optical channel. Additionally, it should be understood that the light source may be turned off during the exposure time of the frame. To achieve a sufficient frame rate, the gain of the imaging device may be increased to compensate for the shorter exposure time due to the presence of the intermission time. Thereby, the duration of each frame including the exposure time and the intermission time may be maintained.

[0017] An imaging device configured not to record light from the scene may be achieved by physically blocking light from reaching the imaging device (e.g., through an aperture) and / or by configuring the imaging device not to generate or send any data when exposed to light.

[0018] Similarly, the exposure time of the frame may be represented by exposing the imaging device to light and / or by configuring the imaging device to generate or send data when exposed to light.

[0019] Further envision that the synchronization can further include turning on the light source during at least a portion of the at least one intermittent time of every Kth frame of the frame sequence, where K includes a value between 1 and 10. Alternatively, the synchronization can further include turning on the light source during at least a portion of the at least one intermittent time of corresponding frames of subsequent frame groups in the frame sequence. The frame sequence can include a plurality of such subsequent frame groups, which are separated by frame intervals during which the light source is turned off for the entire duration of the frame. In other words, the light source can be turned off during the at least one intermittent time (and thus the entire frame) of every Mth frame, where M includes a value between 2 and 10.

[0020] Processing the acquired data can include processing data related to each frame of the first subset. Thus, it is possible to process only the data related to the frames during which the light source is off. Thereby, a more efficient method for alleviating or eliminating the common optical channel crosstalk problem in terms of the demand for processing resources is achieved. Discarding the data related to the second frame subset can preferably be performed before performing any kind of compression on the frames in the frame sequence.

[0021] The light source can be the laser indicator of the geodetic instrument or the light source of the electronic distance measurement unit of the geodetic instrument.

[0022] According to a second aspect, there is provided a geodetic instrument. The geodetic instrument includes: a light source for assisting a user in aiming at a target in a scene; and an imaging device configured to capture an image of the scene using a frame sequence. The light source is arranged to emit light pulses towards the target / scene. The frames of the frame sequence include an exposure time during which the imaging device is exposed to light emission from the scene. The imaging device and the light source share a common optical channel within the geodetic instrument. The geodetic instrument further includes a processing unit configured to: synchronize the emission of these light pulses with the frame sequence to obtain data from images in which these light pulses are absent, and process the acquired data to measure the scene. It is envisioned that the geodetic instrument can further include a processing unit configured to: synchronize the emission of these light pulses with the frame sequence to obtain data from images in which these light pulses are absent, and process the acquired data to measure, observe, monitor, and / or view the scene.

[0023] The frame sequence can include a first subset of frames, and the geodetic instrument is further configured to turn off the light source during the exposure time of each frame of the first subset.

[0024] The frame sequence can further include a second subset of frames, where the first subset and the second subset are mutually exclusive subsets, and the geodetic instrument is further configured to turn on the light source during at least a portion of the exposure time of each frame of the second subset.

[0025] The frame sequence may include frames from the first subset, the frames being interleaved with one or more frames from the second subset. Alternatively, the frame sequence may include frames from the second subset, the frames being interleaved with one or more frames from the first subset.

[0026] The geodetic instrument may further be configured to discard data related to each frame of the second subset.

[0027] The second subset may include every Nth frame of the frame sequence, where N ranges from 2 to 10. It is further contemplated that the first subset may include every Nth frame of the frame sequence, where N ranges from 2 to 10.

[0028] The frames of the frame sequence may further include at least one intermittent time period during which the imaging device is configured not to record light from the scene, and the geodetic instrument is further configured to cause the light source to be turned on during at least a portion of the at least one intermittent time period for at least some of the frames of the frame sequence or the first subset.

[0029] The geodetic instrument may further be configured to cause the light source to be turned on during at least a portion of the at least one intermittent time period for every Kth frame of the frame sequence, where K ranges from 1 to 10.

[0030] The processing unit may further be configured to process data related to each frame of the first subset.

[0031] The light source may be the light source of an electronic distance measurement unit or the laser indicator of the geodetic instrument. The light source may be a visible light semiconductor laser or LED with a rating in the mW range. Some pulse frequencies will be discussed in the detailed description of the present disclosure. However, it is contemplated that the pulse frequency of the light source may be in the MHz range. Thus, the pulse train may be synchronized with the frames in the frame sequence. Accordingly, within a single frame of the frame sequence, the laser may be turned on and off multiple times, depending on the frequency. A high enough frequency may cause the laser dot viewed by an observer (the observer directly views the laser dot or views the laser dot via a display) to appear as a continuous laser dot.

[0032] The imaging device may have a frame rate of at least 10 Hz (such as at least 15 Hz).

[0033] According to a third aspect of the present disclosure, a processing unit of a geodetic instrument is provided. The processing unit is adapted to determine the distance and / or direction to a target in a scene. The processing unit may further be configured to operate the geodetic instrument according to the method defined in any of the embodiments described above with respect to the first aspect.

[0034] Features described with respect to one aspect may also be incorporated in other aspects, and the advantages of such features apply to all aspects incorporating the same.

[0035] Other objects, features and advantages of the inventive concept will become apparent from the following detailed disclosure, from the appended claims as well as from the drawings.

[0036] In general, unless otherwise explicitly defined herein, all terms used in the claims should be interpreted according to their ordinary meaning in the relevant art. Further, in this document, the use of terms such as “first,” “second,” and “third,” etc. does not denote any order, quantity, or importance, but rather is used to distinguish one element from another. Unless otherwise explicitly stated, all statements of “a / an / the [element, device, component, apparatus, step, etc.]” should be construed open - endedly as referring to at least one instance of the element, device, component, apparatus, step, etc. Unless explicitly stated, the steps of any method disclosed herein are not necessarily to be performed in the exact order disclosed. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] With reference to the drawings, the above - mentioned and additional objects, features and advantages of the inventive concept will be better understood from the following illustrative but non - limiting detailed description of the inventive concept, in which:

[0038] Figure 1a An embodiment of a geodetic instrument is schematically shown;

[0039] Figure 1b An embodiment of a geodetic instrument is schematically shown;

[0040] Figure 1c An embodiment of a geodetic instrument is schematically shown;

[0041] Figure 2a A frame sequence according to an embodiment is schematically shown;

[0042] Figure 2b A frame sequence according to an embodiment is schematically shown;

[0043] Figure 2c A frame sequence according to an embodiment is schematically shown;

[0044] Figure 3a A frame sequence according to an embodiment is schematically shown;

[0045] Figure 3b A frame sequence according to an embodiment is schematically shown;

[0046] Figure 3cSchematically shows a frame sequence according to an embodiment; and

[0047] Figure 4 Schematically shows a method for operating a geodetic instrument in a flow chart.

[0048] These figures are not necessarily drawn to scale and generally only show parts necessary to illustrate the inventive concept, where other parts may be omitted or merely suggested. Detailed Description

[0049] The inventive concept generally relates to the optical functions of a geodetic instrument, such as for measurement. In particular, the operation of a geodetic instrument according to the inventive concept is advantageous in a geodetic instrument having a specific optical setup, such as will be described further below with reference to Figure 1a , Figure 1b and Figure 1c Further described. Such a geodetic instrument will allow different functional modules to be easily integrated into the geodetic instrument through a simple interface, such that multiple optical paths overlap at least along the lens arrangement. Thus, the optical paths can have the same lenses of a common lens arrangement. More specifically, the separation of the optics of the lens arrangement from the distance measurement unit, the imaging unit, and the tracker provides flexibility in the design of the geodetic instrument, where a clear and simple interface allows such functional modules to be added or replaced.

[0050] Briefly, in one embodiment, the geodetic instrument includes a lens arrangement; an imaging unit configured to obtain an image of at least a part of a target; a light source arranged to assist a user in aiming at a target in a scene and / or arranged as part of a distance measurement unit (or electronic distance measurement unit, EDM unit) configured to measure the distance to the target along the optical axis of the light source; and a beam splitter / combiner. The beam splitter / combiner combines partial optical paths of the imaging unit and the light source. Thus, the light source can be part of the distance measurement unit. However, it will be understood that the light pulses emitted by the light source are not necessarily distance measurement light or distance measurement pulses. Although this disclosure relates to distance measurement light, it should be understood that this disclosure also includes embodiments where the light source is configured to assist a user in aiming at a target. In other words, the light source can be used as a laser pointer.

[0051] In particular, the optical paths are arranged such that the optical axis of the imaging unit and the optical axis of the light source are coaxial with the optical axis of the lens arrangement at least between the lens arrangement and the beam splitter / combiner, i.e., parallel and overlapping.

[0052] Figure 1aShows the components of a geodetic instrument 100A according to an embodiment. The geodetic instrument 100A includes a lens arrangement 110, an imaging unit 120, a light source 140, and a beam splitter / combiner 150.

[0053] The lens arrangement 110 is provided for observing an object, such as a reflector, as a target. The optical arrangement 110 includes at least one focusing lens element 116 that is movably arranged for focusing to observe the object. For example, the focusing lens element can be a single or compound focusing lens that is manually or automatically moved to produce a focused / defocused image of the object that can be viewed through an eyepiece (which constitutes a simple imaging unit). The lens arrangement 110 can be part of a telescope known in the field of measurement and can include multiple optical lenses (such as lenses 114 and 116) to enable focusing and zooming.

[0054] The imaging unit 120 is configured to obtain an image of at least a part of the object observed by the lens arrangement 110. The imaging unit 120 can be a simple lens or an eyepiece (also known as an ocular) that enables an image to be obtained in the user's eye. Alternatively, the imaging unit 120 can be a combination of an electronic imaging device, a microdisplay, and an eyepiece, such that the image can be conventionally recorded and viewed with the eye through the eyepiece. Preferably, the imaging unit 120 is an electronic imaging device, such as an array of two-dimensional sensor elements capable of generating image information (the number of pixels in the image information generally corresponding to the number of array elements), such as a charge-coupled device (CCD) camera or a metal oxide semiconductor (CMOS) camera. Such a sensor array can consist of 1000×1000 or more sensor elements to generate a digital image with 10 6 image pixels (1 megapixel) or more image pixels. However, smaller sensor arrays are also feasible, such as a sensor array consisting of, for example, 480×750 sensor elements. Alternatively, the sensor array can consist of APDs forming an avalanche photodiode (APD) array. The light source 140 can be configured to measure the distance to an object along the optical axis that constitutes the optical measurement axis of the light source 140. For example, the light source 140 can use electronic distance measurement (EDM) to obtain a measurement value regarding the distance. In one example, the light source includes a distance measurement unit 140 that includes a coherent light source, such as an infrared laser or another suitable laser (which emits, for example, in the red wavelength range). The distance measurement unit can preferably include a fast reflectorless operating EDM. Conventionally, collimated light is emitted from the measuring device 100A along a radial direction to perform distance measurement by using a pulse method or a phase method known in the art. Further, the intensity of the received EDM signal (i.e., the backscattered signal of electro-optical distance measurement) can also be used to obtain information regarding the distance to the reflecting object.

[0055] In Figure 1a it, a beam splitter / combiner 150 is provided and configured to combine a partial optical imaging path of the imaging unit 120 with a partial optical distance measurement path of the light source 140 such that the optical axis of the imaging unit 120 and the optical axis of the light source 140 are coaxially arranged with the optical axis of the lens arrangement at least between the lens arrangement 110 and the beam splitter / combiner 150.

[0056] The optical axis can be regarded as an imaginary line that defines the path along which light propagates through the system, up to a first-order approximation. For a system consisting of simple lenses and mirrors, the optical axis passes through the center of curvature of each surface and coincides with the axis of rotational symmetry. The optical path can be regarded as the path taken by light as it passes through an optical system (such as the lenses of the lens arrangement 110). The optical path (beam path) is typically limited to the three-dimensional volume that is rotationally symmetric about the optical axis within which light can travel.

[0057] The optical axis of the imaging unit 120 is shown by a dashed line and the optical axis of the light source 140 is shown by a dotted line. These lines do not give a preferred direction (optical reciprocity), but it can be considered that the light to be imaged on the imaging unit 120 travels from left to right. The beam splitter / combiner 150 combines these lines so that the optical axes overlap in the left-hand portion of the beam splitter / combiner 150 in Figure 1a . Since these axes are parallel and overlap with each other (i.e., are coaxial), and additionally are parallel and overlap with the optical axis of the lens arrangement 110, these axes are also regarded as being coaxially arranged with respect to the optical axis of the lens arrangement 110.

[0058] In particular, the optical setup, and especially the beam splitter / combiner 150, is chosen such that the optical axis of the lens arrangement 110 corresponds to the overlapping optical axes of the imaging unit 120 and the light source 140 between the beam splitter / combiner 150 and the lens arrangement 110 and along the lens arrangement 110, such that the light traveling in the respective optical paths is affected by the lenses 114 and 116 of the lens arrangement. Since the optical axes of the light source 140 and the imaging unit 120 partially overlap, the optical paths of these units (i.e., the optical imaging path and the optical distance measurement path) also partially overlap when combined by the beam splitter / combiner 150. In this device, the centers of the two-dimensional sensor element array acting as the imaging unit and the tracking unit do not need to coincide with the optical axis of the lens arrangement 110. For example, in a calibration step, the center can be defined as the point where the optical axis coincides with the array on the two-dimensional sensor element array.

[0059] It will be understood that the coaxial arrangement of two or more optical axes is basically a theoretical assumption, because in practice, the optical axes usually do not completely overlap and point in exactly the same direction, but will overlap within a certain small error range. Therefore, for the sake of explanation, it is assumed that an axial deviation less than + / -0.2° is still coaxial. Typical alignment errors are on the order of + / -0.1°, which can be corrected by software in subsequent calibrations.

[0060] Viewed from different angles Figure 1a , according to the principle of optical reciprocity, the light coming from Figure 1a the left side and thus entering the lens arrangement 110 from the left side will be separated by the beam splitter / combiner 150. Therefore, it is technically the same to describe one optical path being split into two optical paths by the beam splitter / combiner (viewed from left to right) and to describe two optical paths being combined by the beam splitter / combiner (viewed from right to left).

[0061] In a simple case, a semi-transparent mirror can be used as the beam splitter / combiner to split the incident light into two parts (e.g., 50:50), one part reaching the imaging unit 120 and the other part reaching the light source 140. Then, the unwanted distance measurement light in the imaging channel can be filtered out before hitting the imaging unit 120. However, in practice, dichroic mirrors or dichroic prisms are used, that is, mirrors or prisms that are transparent to one wavelength range and reflective to another wavelength range. This wavelength selectivity can be achieved by using dichroic filters / membranes based on the principle of thin-film interference. Therefore, using dichroic mirrors or dichroic prisms respectively allows a larger percentage of reflected light and transmitted light to be used.

[0062] Therefore, according to the direction and wavelength of the light, the beam splitter / combiner 150 is configured not only to combine the light beams, but also to split the light reflected from the object passing through the lens arrangement into imaging light along the optical imaging path and distance measurement light along the optical distance measurement path.

[0063] As can be seen from the above, the optical path and the optical axis are independent of the direction of light travel, so "split" and "combine" are only for better explaining the optical layout. In particular, the imaging unit in these examples only receives light and does not emit any light, so the beam splitter / combiner does not combine the light from the imaging unit and the light source, but it is configured with an optical function that can do so, because the light entering the geodetic instrument through the lens arrangement is split into different channels by the beam splitter / combiner. In other words, the optical function of the beam splitter / combiner is to combine different paths from its right side to overlap on its left side.

[0064] In one example, the laser diode of the light source 140 can emit light in the red range of about 660 nm (or 635 nm), and the imaging unit 120 can image a scene including an object that reflects visible wavelengths. Thus, if a dichroic mirror with a cut-off wavelength of about 620 nm (i.e., reflecting wavelengths greater than 620 nm) (alternatively, a notch filter that blocks light around 635 nm) is provided, distance measurement and imaging can be achieved with little to no intensity loss in different channels. Using a dichroic prism design further allows the camera chip of the imaging unit to be directly glued to a part of the prism, resulting in a highly compact structure that is largely insensitive to temperature changes and external shocks, while mechanical components for attaching and aligning the camera chip can be saved.

[0065] If the lens 116 in the lens arrangement 110 facing the beam splitter / combiner has a convex surface (e.g., a plano-convex lens or a bi-convex lens facing the beam splitter / combiner), additional reliability of the measurement of the measuring device can be achieved. As a result, the reflection of the distance measurement light from the light source 140 from this lens is not reflected back to the distance measurement unit 140, thus avoiding crosstalk that may cause signals not from the actual target (object) to be detected. In addition, an anti-reflection coating on the lens of the lens arrangement can also reduce crosstalk. When using a prism as the beam splitter / combiner 150, the intermediate focus should be placed outside rather than inside the prism, and the (multiple) prism surfaces where light is incident can be slightly inclined relative to the orthogonal direction such that the light is not incident on it completely orthogonally. In addition, an air gap can be provided between the respective prisms in place to achieve total internal reflection.

[0066] In Figure 1b an additional embodiment of a geodetic instrument is provided, which is further constructed on top of Figure 1a the geodetic instrument 100A. Specifically, the geodetic instrument 100B includes the same elements as the geodetic instrument 100A and additionally includes a tracker 130.

[0067] The tracker 130 is configured to track an object, such as a prism reflector, by preferably using infrared light with a wavelength of 850 nm (or 810 nm). It can be directly understood from Figure 1b that Figure 1a the beam splitter / combiner 150 of Figure 1bThe beam splitter / combiner 150' is configured to combine a part of the optical tracking path of the tracker 130, a part of the optical imaging path of the imaging unit 120, and a part of the light source path of the light source 140, such that the optical axes of the tracker, the imaging unit, and the light source are coaxially arranged with the optical axis of the lens arrangement 110 at least between the lens arrangement and the beam splitter / combiner 150'. Accordingly, the lens arrangement 110 is shared by the tracking, aiming assistance, and / or distance measurement and imaging functions.

[0068] More specifically, in Figure 1b , the optical axis of the tracker 130 is shown by the dotted line 131, the optical axis of the imaging unit 120 is shown by the dash-dotted line 121, and the optical axis of the light source 140 is shown by the dashed line 141. In Figure 1b , it is schematically shown how the light of these optical axes is reflected and transmitted by the beam splitter / combiner 150' to coincide with the optical axis 111 of the lens arrangement 110.

[0069] Figure 1b The prism system shown is a multi-channel prism. In particular, the prism system includes two prisms having a wedge shape. In a preferred embodiment, the beam splitter / combiner 150' includes at least two wedge prisms and a wavelength-selective surface. A wavelength-selective surface is any surface that reflects different wavelengths in different ways. In the above example of a dichroic mirror (or a similar dichroic prism), the dichroic mirror (or dichroic prism) may also include a wavelength-selective surface. The more optical paths that need to be combined, the more prisms or mirrors (or combinations thereof) need to be provided. Thus, in a preferred embodiment having three functional modules (such as the tracker 130, the imaging unit 120, and the light source 140), the prism system is composed of two dichroic prisms having surfaces similar to a dichroic mirror.

[0070] Those skilled in the art will recognize that instead of Figure 1b the two dichroic prisms shown, two dichroic mirrors may also be used. Thus, similar to Figure 1a , the optical axes of the tracker, the light source, and the imaging unit can be coaxially arranged with the optical axis of the lens arrangement 110 to the left of the beam splitter / combiner 150'.

[0071] Figure 1c Elements of another example of a geodetic instrument 100C are shown. The geodetic instrument 100C includes a lens arrangement 110, an imaging unit 120, a tracker 130, and a beam splitter / combiner 150. The geodetic instrument 100C corresponds to the geodetic instrument 100A, but the light source 140 is replaced by a tracker 130, where the details of the tracker 130 have been discussed with respect to Figure 1b .

[0072] Now referring to Figure 2a, shows the frame sequence 100a. The shown frame sequence 100a includes frames 102a - 118a. However, it should be understood that the frame sequence may include fewer or more frames. Each frame in the frame sequence 100a includes an exposure time. The first frame 102a includes the exposure time 120a. During the capture of the first frame 102a, the light source is turned off for the entire duration of the frame (i.e., the exposure time 120a), thereby allowing the imaging device to obtain data from an image without light pulses corresponding to the first frame 102a.

[0073] In contrast, during the capture of the second frame 104a, the light source is turned on 124a for at least a portion of the frame (i.e., at least a portion of the duration of the exposure time 122a of the second frame 104a). In the shown frame sequence 100a, the light source is turned on 124a for the entire duration of the second frame 104a (i.e., during the duration of the exposure time 122a). Thus, the image corresponding to the second frame 104a may include data related to light pulses.

[0074] The frame sequence 100a includes frames belonging to a first subset of frames, wherein the light source is turned off during the exposure time of each frame in the subset. In the shown frame sequence 100a, the frames 102a, 106a, 110a, 114a, and 118a are included in the first subset of frames. Additionally, the frames 104a, 108a, 112a, and 116a are included in a second subset of frames, wherein the light source is turned on for at least a portion of each frame in the subset. Thus, synchronization of light pulse emission with the frame sequence 100a is achieved.

[0075] The frame rate of the frame sequence 100a and / or the peak power of the light source can affect how the synchronization of light pulse emission is to be performed. For example, in the shown embodiment, a frame sequence with a frame rate of 50 Hz (i.e., each frame has a duration of 20 ms) will have 25 frames belonging to the first subset of frames and 25 frames belonging to the second subset of frames per second. Since data from images without light pulses is to be processed, only the data related to the first subset of frames will be utilized, thereby compensating for possible crosstalk. Thus, a display unit configured to display the scene captured by the imaging device can display 25 unique frames per second. For this purpose, the term "unique" will be understood as a frame in the frame sequence that is not created by duplicating or inserting another frame in the frame sequence. Additionally, even if each frame in the first subset is duplicated to create a modified frame sequence with 50 frames per second, the modified frame sequence will only include 25 unique frames, each frame being displayed continuously twice for a total duration of 40 ms. Thus, the perceived frame rate of such a modified frame sequence displayed on the display device will be 25 frames per second.

[0076] In addition, as in the example set forth in the previous paragraph, the light source can be switched back and forth between an operating state and a non-operating state, with each operating cycle being 20 ms and each non-operating cycle also being 20 ms. Thus, the duty cycle of the light source can be said to be 50%. Depending on the peak power of the light source, when directly viewed by a user, such switching can still form a perceived continuous light spot on the target irradiated by the light pulse.

[0077] Frame sequence 100a includes a first frame subset that includes every second frame of the frame sequence. Correspondingly, the second frame subset also includes every second frame of the frame sequence. However, it is contemplated that the first frame subset can include every Nth frame of the frame sequence, where N ranges from 2 to 10. It is also contemplated that the second subset can include every Nth frame of the frame sequence, where N ranges from 2 to 10.

[0078] Now referring Figure 2b , frame sequence 100b is shown. The shown frame sequence 100b includes frames 102b - 118b, however it should be understood that the frame sequence can include fewer or more frames. Each frame in frame sequence 100b includes an exposure time. The first frame 102b includes exposure time 120b. During the capture of the first frame 102b, the light source is turned off for the entire duration of the frame (i.e., exposure time 120b), thus allowing the imaging device to obtain data from an image without light pulses corresponding to the first frame 102b.

[0079] In contrast, during the capture of the third frame 106b, the light source is turned on 124b for at least a portion of the frame (i.e., at least a portion of the duration of the exposure time 122b of the third frame 106b). In the shown frame sequence 100b, the light source is turned on 124b for the entire duration of the third frame 106b (i.e., during the duration of the exposure time 122b). Thus, the image corresponding to the third frame 104b can include data related to the light pulse.

[0080] Frame sequence 100b includes frames belonging to the first frame subset, where the light source is turned off during the exposure time of each frame in the subset. In the shown frame sequence 100b, frames 102b, 104b, 108b, 110b, 114b, and 116b are included in the first frame subset. Additionally, frames 106b, 112b, and 118b are included in the second frame subset, where the light source is turned on for at least a portion of each frame in the subset. Thus, synchronization of the light pulse emission with frame sequence 100b is achieved.

[0081] In frame sequence 100b, two-thirds of the frames belong to a first frame subset, while one-third of the frames belong to a second frame subset. Thus, a frame sequence with a frame rate of, for example, 60 Hz will have 40 frames per second belonging to the first frame subset and 20 frames per second belonging to the second frame subset. Thus, 40 images without light pulses can be captured per second. Similarly, the light source can be repeatedly switched between an on state and an off state, with each on period being 20 ms and each off period being 40 ms. Thus, the duty cycle of the light source can be said to be approximately 33%. Depending on the peak power of the light source, when directly viewed by a user, such switching can still form a perceived continuous light spot on the target illuminated by the light pulses. Further, the video generated based on the data obtained from the first frame subset does not suffer from crosstalk.

[0082] The frame sequence 100b includes a second frame subset that includes every third frame of the frame sequence. However, it is contemplated that the second frame subset can include every Nth frame of the frame sequence, where N ranges between 2 and 10.

[0083] Now referring Figure 2c , frame sequence 100c is shown. The shown frame sequence 100c includes frames 102c - 118c, however, it should be understood that the frame sequence can include fewer or more frames. Each frame in the frame sequence 100c includes an exposure time. The third frame 106c includes an exposure time 120c. During the capture of the third frame 106c, the light source is turned off for the entire duration of the frame (i.e., exposure time 120c), thereby allowing the imaging device to obtain data from an image without light pulses corresponding to the third frame 102c (thus avoiding crosstalk).

[0084] In contrast, during the capture of the first frame 102c, the light source is turned on 124c for at least a portion of the frame (i.e., at least a portion of the duration of the exposure time 122c of the first frame 102c). In the shown frame sequence 100c, the light source is turned on 124c for the entire duration of the first frame 102c (i.e., during the duration of the exposure time 122c). Thus, the image corresponding to the first frame 104c can include data related to the light pulses.

[0085] The frame sequence 100c includes frames belonging to a first frame subset, where the light source is turned off during the exposure time of each of the frames. In the shown frame sequence 100b, frames 106c, 112c, and 118c are included in the first frame subset. Additionally, frames 102c, 104c, 108c, 110c, 114c, and 116c are included in the second frame subset, where the light source is turned on for at least a portion of each of the frames. Thus, synchronization of the light pulse emission with the frame sequence 100c is achieved.

[0086] In frame sequence 100c, one-third of the frames belong to a first frame subset, while two-thirds of the frames belong to a second frame subset. Thus, a frame sequence with, for example, a 60 Hz frame rate will have 20 frames per second belonging to the first frame subset and 40 frames per second belonging to the second frame subset. Thus, 20 images without light pulses can be captured per second. Similarly, the light source can be repeatedly switched between an on state and an off state, with each on cycle being 40 ms and each off cycle being 20 ms. Thus, the duty cycle of the light source can be said to be approximately 67%. Depending on the peak power of the light source, when directly viewed by a user, this switching can still form a perceived continuous light spot on the target illuminated by the light pulses.

[0087] Frame sequence 100c includes a first frame subset that includes every third frame of the frame sequence. However, it is contemplated that the first frame subset could include every Nth frame of the frame sequence, where N ranges between 2 and 10.

[0088] Reference Figures 2a to 2c , in each respective frame sequence, one frame from the first subset is interleaved with one or more frames from the second subset, or one frame from the second subset is interleaved with one or more frames from the first subset. Thereby, synchronization of the light pulse emission with the frame sequence is achieved, allowing data to be obtained from images without light pulses. As a result, the quality of the image sequence or video provided by the instrument is improved since the occurrence of crosstalk in the optical channel shared by the imaging device and the light source is avoided or at least reduced.

[0089] Now reference Figure 3a , frame sequence 180a is shown. The shown frame sequence 180a includes frames 130a - 146a, however it should be understood that the frame sequence could include fewer or more frames. Each frame in frame sequence 180a includes an exposure time and at least one intermission time. The first frame 130a includes exposure time 150a and intermission time 152a. During the capture of the first frame 130a, the light source is turned off for the entire duration of the exposure time 150a, thereby allowing the imaging device to obtain data from the image without light pulses corresponding to the first frame 130a. Additionally, the light source is turned on during at least a portion of the intermission time 152a. Thus, the light emitted by the light source is not captured by the imaging device in the first frame 130a, but the light pulses can still form light spots on the target to assist the user in aiming at the target.

[0090] In the shown frame sequence 180a, all remaining frames 132a - 146a of the frame sequence 180a are configured according to the description of the first frame 130a in the previous paragraph, i.e., each frame includes an exposure time and at least one intermission time, the light source is turned off during that exposure time, and the light source is turned on during at least a portion of that at least one intermission time.

[0091] Since, compared with the embodiments described in connection with Figures 2a to 2c the exposure time is shorter relative to the duration of each frame, the gain of the imaging device is increased to obtain sufficient images. Additionally, it can be noted that the light source is not turned on during the exposure time of any frame. Thus, the imaging device capturing images via the frame sequence 180a will not acquire images where light is emitted by the light source.

[0092] Still referring to Figure 3a , light pulses emitted by the light source are synchronized with the frame sequence 180a such that the light source is turned on during at least a portion of at least one intermission time of each frame within the frame sequence 180a.

[0093] Now referring to Figure 3b , a frame sequence 180b is shown. The frame sequence 180b shown includes frames 130b - 146b, however, it should be understood that the frame sequence can include fewer or more frames. Each frame in the frame sequence 180b includes an exposure time and at least one intermission time. The third frame 134b includes an exposure time 150b and an intermission time 152b. During the capture of the third frame 134b, the light source is turned off for the entire duration of the exposure time 150b, thereby allowing the imaging device to obtain data from an image where no light pulse is present corresponding to the third frame 134b. Additionally, the light source is turned on during at least a portion of the intermission time 152b. Thus, the light emitted by the light source is not captured by the imaging device in the third frame 134b, but the light pulse can still form a light spot on the target to assist the user in aiming at the target.

[0094] In the frame sequence 180b shown, every third frame is configured according to the description of the third frame 134b in the previous paragraph, i.e., frames 134b, 140b, 146b each include an exposure time and at least one intermission time, the light source is turned off during that exposure time, and the light source is turned on during at least a portion of that at least one intermission time. To this end, it is further contemplated that the light source can be turned on during at least a portion of at least one intermission time of each frame, every other frame, every third frame, every fourth frame, every fifth frame, or more generally, every Kth frame, where K ranges from 1 to 10.

[0095] Now referring to Figure 3c, shows a frame sequence 180c. The shown frame sequence 180c includes frames 130c - 146c. However, it should be understood that the frame sequence may include fewer or more frames. Each frame in the frame sequence 180c includes an exposure time and at least one intermission time. The first frame 130c includes an exposure time 150c and an intermission time 152c. During the capture of the first frame 130c, the light source is turned off for the entire duration of the exposure time 150c, thereby allowing the imaging device to obtain data from an image without light pulses corresponding to the first frame 130c. Additionally, the light source is turned on during at least a portion of the intermission time 152c. Thus, the light emitted by the light source is not captured by the imaging device in the first frame 130c, but the light pulses can still form light spots on the target to assist the user in aiming at the target.

[0096] In the shown frame sequence 180c, for every third frame in the frame sequence 180c, the light source is turned off not only during the exposure time of the frame but also during the intermission time of the frame. Thus, each of the frames 134b, 140b, 146b includes an exposure time and at least one intermission time, with the light source turned off during the exposure time of the frame and also during the at least one intermission time of the frame. In contrast, each of the frames 130c, 132c, 136c, 138c, 142c, 144c includes an exposure time and at least one intermission time, with the light source turned off during the exposure time of the frame and the light source turned on during the at least one intermission time of the frame. For this purpose, it is further contemplated that the light source can be turned off during the exposure time and intermission time of every other frame, every third frame, every fourth frame, every fifth frame, or more generally, every Mth frame (i.e., during the entire duration of the frame), where M ranges between 2 and 10.

[0097] Now reference will be made to Figure 4 Describe a method for operating a geodetic instrument according to the inventive concept. The geodetic instrument includes a light source and an imaging device, the light source being used to assist the user in aiming at a target in a scene, wherein the imaging device and the light source share a common optical path within the geodetic instrument. The method includes: causing the light source to emit 450 light pulses towards the target; causing the imaging device to capture 452 an image of the scene using a frame sequence, wherein the frames of the frame sequence include an exposure time during which the imaging device is exposed to light from the scene; synchronizing 454 the emission of the light pulses with the frame sequence to obtain data from an image without light pulses; and processing 456 the obtained data to measure the scene.

[0098] For clarity and simplicity, the method will be described in terms of "steps". It should be emphasized that the steps are not necessarily processes defined by time or separated from each other, and more than one "step" can be executed simultaneously in a parallel manner.

[0099] As will be readily understood by those skilled in the art, many modifications and variations can be designed in view of the above description of the principles of the inventive concept. It is intended that all such modifications and variations be regarded as within the scope of the inventive concept as defined in the appended patent claims.

Claims

1. A method for operating a geodetic instrument, the geodetic instrument including a light source and an imaging device, the light source being used to assist a user in aiming at a target in a scene, wherein, the imaging device and the light source share a common optical channel within the geodetic instrument, and the method includes: causing the light source to emit light pulses towards the target; causing the imaging device to capture an image of the scene using a frame sequence, wherein the frames of the frame sequence include an exposure time during which the imaging device is exposed to light from the scene, and wherein the frame sequence includes a first subset of frames; synchronizing the emission of the light pulses with the frame sequence to obtain data from images in which the light pulses are absent, wherein the synchronization includes causing the light source to be in a non-operating state during the exposure time of each frame of the first subset of frames; processing data related only to each frame of the first subset of frames; and causing data related only to each frame of the first subset of frames to be displayed as frames on a display.

2. The method according to claim 1, wherein, the frame sequence further includes a second subset of frames, the first subset of frames and the second subset of frames being mutually exclusive subsets, and the synchronization further includes: causing the light source to be in an operating state during at least a part of each frame of the second subset of frames.

3. The method according to claim 2, wherein, the frame sequence includes frames from the first subset of frames that are interleaved with one or more frames from the second subset of frames, or wherein, the frame sequence includes frames from the second subset of frames that are interleaved with one or more frames from the first subset of frames.

4. The method according to claim 2 or 3, further including: discarding data related to each frame of the second subset of frames.

5. The method according to any one of claims 2 to 4, wherein, the second subset of frames includes every Nth frame of the frame sequence, where N ranges from 2 to 10.

6. The method according to claim 1, wherein, the frame further includes at least one intermittent time during which the imaging device is configured not to record light from the scene, and the method further includes: for at least some frames of the frame sequence, causing the light source to be in an operating state during at least a part of the at least one intermittent time.

7. The method according to claim 1, wherein, the frame further includes at least one intermittent time during which the imaging device is configured not to record light from the scene, and the method further includes: for at least some frames of the first subset of frames, causing the light source to be in an operating state during at least a part of the at least one intermittent time.

8. The method according to claim 6 or 7, wherein, the synchronization further includes causing the light source to be in the operating state during at least a part of the at least one intermittent time of every Kth frame of the frame sequence, where K ranges from 1 to 10.

9. The method according to any one of the preceding claims, wherein, the light source is a laser indicator of the geodetic instrument or a light source of an electronic distance measurement unit of the geodetic instrument.

10. A geodetic instrument, including: A light source for assisting a user in aiming at a target in a scene, the light source being arranged to emit light pulses towards the target / scene; An imaging device configured to capture an image of the scene using a frame sequence, wherein the frames of the frame sequence include an exposure time during which the imaging device is exposed to light emission from the scene, and wherein the frame sequence includes a first subset of frames; wherein the imaging device and the light source share a common optical channel within the geodetic instrument, and A processing unit configured to: Synchronize the emission of the light pulses with the frame sequence to obtain data from images in which the light pulses are absent, wherein the synchronization includes keeping the light source in a non-operating state during the exposure time of each frame of the first subset of frames, Process data related only to each frame of the first subset of frames, and Cause data related only to each frame of the first subset of frames to be displayed as frames on a display.

11. The geodetic instrument according to claim 10, wherein, The frame sequence further includes a second subset of frames, the first subset of frames and the second subset of frames being mutually exclusive subsets, and the geodetic instrument is further configured to keep the light source in an operating state during at least a part of the exposure time of each frame of the second subset of frames.

12. The geodetic instrument according to claim 11, wherein, The frame sequence includes frames from the first subset of frames that are interleaved with one or more frames from the second subset of frames, or wherein the frame sequence includes frames from the second subset of frames that are interleaved with one or more frames from the first subset of frames.

13. The geodetic instrument according to claim 11 or 12, further configured to discard data related to each frame of the second subset of frames.

14. The geodetic instrument according to any one of claims 11 to 13, wherein, The second subset of frames includes every Nth frame of the frame sequence, where N ranges from 2 to 10.

15. The geodetic instrument according to claim 10, wherein, The frames of the frame sequence further include at least one intermittent time during which the imaging device is configured not to record light from the scene, and the geodetic instrument is further configured to keep the light source in an operating state during at least a part of the at least one intermittent time for at least some frames of the frame sequence.

16. The geodetic instrument according to claim 10, wherein, The frames of the frame sequence further include at least one intermittent time during which the imaging device is configured not to record light from the scene, and the geodetic instrument is further configured to keep the light source in an operating state during at least a part of the at least one intermittent time for at least some frames of the first subset of frames.

17. The geodetic instrument according to claim 15 or 16, further configured to keep the light source in the operating state during at least a part of the at least one intermittent time of every Kth frame of the frame sequence, wherein, K ranges from 1 to 10.

18. A geodetic instrument according to any one of claims 10 to 17, wherein, the light source is a light source of an electronic distance measurement unit of the geodetic instrument or a laser pointer of the geodetic instrument.

19. A processing unit of a geodetic instrument, the processing unit being adapted to determine a distance and / or a direction to a target in a scene, wherein, the processing unit is configured to operate the geodetic instrument according to the method defined in any one of claims 1 to 9.

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