Method for operating a geodetic instrument and related geodetic instrument

Abstract

The inventive concept relates to a method for operating a geodetic instrument comprising a light source for assisting a user in aiming at a target in a scene by emitting light pulses to form a light spot at the target, and an imaging device, wherein the imaging device and the light source share a common light channel within the geodetic instrument, the method comprising: capturing a first image of the scene with the light source turned on; obtaining a reference image from at least the first image, wherein influences from the scene are suppressed, the reference image representing crosstalk occurring in the common light channel; capturing a second image with the light source turned on; and processing the second image with the reference image to remove crosstalk from the second image.

Description

TECHNICAL FIELD

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

[0002] Optical instruments, such as geodetic instruments, are often used to measure the position of an object to obtain information such as horizontal and vertical angles and distances. Newer instruments are often provided with electronic imaging devices, e.g. cameras, to acquire digital images of the object.

[0003] Conventional surveying instruments comprise a telescope system for observing an object which can then be imaged on a camera behind the telescope system. Further, such instruments can comprise a distance measuring unit to measure the distance to an object observed by the telescope system. The field of view of the telescope system is often small, e.g. 1 to 2 degrees, and the user has to position the surveying instrument and adjust the optics of the telescope system so 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 functionality is added to the surveying instruments, the complexity of the optical setup and electronic control of the instruments increases. There is therefore a need for more efficient implementation of aiming assistance for geodetic instruments. SUMMARY

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

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

[0007] Furthermore, it has been recognized that geodetic instruments having an imaging device and a light source sharing a common light channel can suffer from crosstalk occurring in the common light channel. Crosstalk is a result of the internal arrangement of features in the geodetic instrument, and thus crosstalk is typically not influenced by external conditions such as changes in ambient light or light reflected by a target in the scene from the light source. This crosstalk can degrade the quality of the image data collected by the imaging device, which can interfere with the measurement, cause features in the image to be difficult to identify, and degrade the user experience. The crosstalk can take any shape and form detected by the imaging device of the geodetic instrument. Furthermore, the crosstalk can be located at any position relative to the pixels of the imaging device. The shape and form of the crosstalk can depend on the beam profile of the light source and / or the optics of the geodetic instrument. Preferably, the 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] A solution to the above problems should preferably allow the imaging device and the light source to perform their respective operations unimpaired or minimally impaired as experienced by a user of the geodetic instrument. Thus, the present disclosure also aims to improve the geodetic instrument with respect to crosstalk occurring in the common light channel.

[0009] According to a first aspect of the inventive concept, these and other objects are all or at least partially achieved by a method for operating a geodetic instrument comprising a light source for assisting a user in aiming at a target in a scene by emitting light pulses to form a light spot at the target, and an imaging device, wherein the imaging device and the light source share a common light channel within the geodetic instrument, the method comprising: capturing a first image of the scene with the light source turned on; obtaining a reference image from at least the first image, wherein influences from the scene are suppressed, the reference image representing crosstalk occurring in the common light channel; capturing a second image with the light source turned on; and processing the second image with the reference image to remove the crosstalk from the second image.

[0010] Thus, a simpler and more compact optical setup is achieved, wherein the light source can operate to assist in aiming at a target in a scene and the imaging device can operate to capture images of the scene without crosstalk occurring in the common light channel shared with the imaging device. The phrase "remove influences from crosstalk" should be understood to include reducing, decreasing, partially removing, or completely removing the mentioned influences crosstalk. Similarly, the phrase "remove influences from the light spot" should be understood to include reducing, decreasing, partially removing, or completely removing the mentioned influences from the light spot.

[0011] In some cases, if the light spot represented by the reference image has, for example, a higher intensity and / or a more extensive distribution compared to the light spot of the second image, the processing by making use of the reference image can overcompensate the influence from the light spot in the second image.

[0012] The obtained reference image represents the crosstalk occurring in the common light channel and can further represent the light spot. Thus, in some embodiments, when processing the second image with the reference image, the influence from the light spot and the crosstalk in the second image can be removed. Thus, the second image will only comprise the influence from the scene. Displaying the second image to a user on a display device will allow the user to view the scene. Furthermore, the user can be able to view the light spot on the object of the scene directly. Here, "directly" is used in the sense that the user can be able to view the light spot on the object of the scene independently of any data generated by the geodetic instrument, i.e. with the user's eyes.

[0013] As will be understood from the present disclosure, in some embodiments, the reference image can simply be obtained by capturing the first image under certain conditions. Such certain conditions can comprise one or more of: aiming the geodetic instrument at the scene, and / or aiming at a target having preferred properties with respect to reflected ambient light and / or light reflected from the light source. Thus, it will be understood that "capturing the first image" and "obtaining the reference image" can in fact be performed in a single action.

[0014] In the context of the present disclosure, a "scene" will be understood to be an object, such as a building, a landscape or other object, that reflects ambient light to the imaging device.

[0015] In the context of the present disclosure, a "target" will be understood to be an object within the scene that can be illuminated by light emitted by the light source. The target can for example be a prism, a building, a landscape or other object.

[0016] In some embodiments, the influence from the light spot in the reference image can be suppressed by aiming the geodetic instrument such that the light source is directed at a target that does not reflect light emitted by the light source back to the imaging device when capturing the first image. The influence from the light spot in the reference image can also be suppressed by aiming the geodetic instrument such that the light source does not generate a corresponding light spot in the first image. The reference image obtained under such conditions can be obtained prior to providing the geodetic instrument to an end user, i.e. as a pre-calibration.

[0017] Thus, the reference image can essentially only represent the crosstalk occurring in the common light channel. Thus, when processing the second image with the reference image, the crosstalk in the second image can be removed.

[0018] In some embodiments, the influence from the scene is suppressed by providing a dark scene or a scene that does not reflect ambient light towards the imaging device. The reference image obtained in such conditions can be obtained before the geodetic instrument is provided to the end user, i.e. as a pre-calibration.

[0019] In some embodiments, the method further comprises capturing a third image of the scene with the light source turned off; and generating a difference image based on the first image and the third image, wherein the influence from the scene is suppressed; wherein the reference image is based on the difference image.

[0020] The difference image of the first image and the third image will preferably remove the influence from the scene, thereby generating an image with only the influence from the crosstalk, and in some cases the influence from the light spot. By aiming the geodetic instrument such that the light source does not generate a light spot when turned on, the influence of the light spot can also be removed from the difference image.

[0021] In some embodiments, the method can further comprise capturing a fourth image of a second scene with the light source turned on; capturing a fifth image of the second scene with the light source turned off; generating a second difference image of the fourth image and the fifth image, wherein the influence from the second scene is suppressed; determining the lowest recorded intensity level of each pixel of the imaging device in the first difference image and the second difference image, wherein the reference image is based on the first difference image and the second difference image using the lowest recorded intensity level determined for each respective pixel of the imaging device.

[0022] Thus, the reference image can be improved by updating the reference image with the lowest recorded intensity level of each pixel of the imaging device in at least the first difference image and the second difference image. The inventors have realized that the lowest recorded intensity level of each pixel of the imaging device in the difference image will result in a more accurate representation of the crosstalk in the geodetic instrument. Over time, additional difference images can be compared and the lowest recorded intensity level of each respective pixel of the imaging device in all difference images, including previously generated difference images and additional difference images, should preferably accurately represent the crosstalk occurring in the common light channel.

[0023] In some embodiments, the method further comprises capturing additional images with the light source turned on, the additional images and the first image forming a plurality of images; wherein obtaining the reference image comprises determining the lowest recorded intensity level of each pixel of the imaging device in the plurality of images; and obtaining the reference image using the lowest recorded intensity level determined for each respective pixel of the imaging device. Each image of the additional images can be an image of a different scene.

[0024] The inventors have realized that the lowest recorded intensity level of each pixel of the imaging device in a plurality of images can provide an accurate estimate of the cross talk in the geodetic instrument. Over time, the lowest recorded intensity level of each respective pixel of the imaging device in all images should preferably accurately represent the cross talk occurring in the common light channel. Preferably, one or more of the plurality of images will capture a dark scene or a scene that does not reflect ambient light towards the imaging device, or at least a partially dark scene or a scene whose some parts do not reflect ambient light towards the imaging device.

[0025] The reference image can be updated even after it has been obtained. An additional image can be captured after the reference image has been obtained, and if the lowest recorded intensity level of a pixel of the imaging device in the additional image is lower than the recorded intensity level of the corresponding pixel in the reference image, the corresponding pixel in the reference image can be updated with the lower recorded intensity level. Likewise, with the necessary modifications, the procedure can be applied to the embodiment of comparing a plurality of difference images.

[0026] According to a second aspect of the inventive concept, these and other objects are all or at least partially achieved by a method for operating a geodetic instrument comprising a light source for assisting a user in aiming at a target in a scene by emitting light pulses to form a light spot at the target, wherein the imaging device and the light source share a common light channel within the geodetic instrument. The method comprises capturing a plurality of images with the light source turned on; determining the lowest recorded intensity level of each pixel of the imaging device in the plurality of images; obtaining a reference image using the lowest recorded intensity level determined for each respective pixel of the imaging device, the reference image representing the cross talk occurring in the common light channel. The method further comprises capturing an image to be processed with the light source turned on; and processing the captured image to be processed with the reference image to remove the cross talk in the image to be processed.

[0027] By processing the captured image to be processed, a processed image can be obtained, wherein the cross talk occurring in the common light channel can be reduced, diminished or removed.

[0028] As discussed in relation to the first aspect, the inventors have realized that the lowest recorded intensity level of each pixel of the imaging device in a plurality of images can provide an accurate estimate of the cross talk in the geodetic instrument. Over time, the lowest recorded intensity level of each respective pixel of the imaging device in all images should preferably accurately represent the cross talk occurring in the common light channel. Preferably, one or more of the plurality of images will capture a dark scene or a scene that does not reflect ambient light towards the imaging device, or at least a partially dark scene or a scene whose some parts do not reflect ambient light towards the imaging device.

[0029] The reference image can be updated even after it has been obtained. An additional image can be captured after the reference image has been obtained, and if the lowest recorded intensity level of a pixel of the imaging device in the additional image is lower than the recorded intensity level of the corresponding pixel in the reference image, the corresponding pixel in the reference image can be updated with the lower recorded intensity level.

[0030] According to a third aspect of the present inventive concept, these objects, as well as others, are all or at least partially met by a geodetic instrument comprising: a light source for assisting a user in aiming at a target in a scene by emitting a light pulse to form a light spot at the target; an imaging device configured to capture images of the scene using a sequence 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 operate the geodetic instrument according to the method as defined in any embodiment of the first and second aspects.

[0031] In some embodiments, the light source is an infrared laser, and the imaging device is an infrared camera or a thermal imaging camera.

[0032] In some embodiments, the light source is a laser in the visible spectrum, and the imaging device is configured to detect visible light.

[0033] The light source can be a light source of an electronic distance measurement unit or a laser pointer of a geodetic instrument. The light source can be a semiconductor laser or an LED with a nominal value in the range of mW.

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

[0035] Other objects, features, and advantages of the present inventive concept will become apparent from the following detailed description, from the appended claims, and from the drawings in which:

[0036] Generally, all terms used herein are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined herein otherwise. Further, the use of the terms "first", "second" and "third", etc. does not imply any order, quantity, or importance, but rather are used to distinguish one element from another. Unless explicitly stated otherwise, any reference to "an" or "a" pertaining to an element of the disclosure should be understood as referring to at least one of the elements of the disclosure. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. BRIEF DESCRIPTION OF DRAWINGS

[0037] The above and additional objectives, features and advantages of the present inventive concept will be better understood through the following illustrative and non-limiting detailed description of the present inventive concept, particularly when read in conjunction with the accompanying drawings, where:

[0038] Fig. 1a schematically illustrates an embodiment of a geodetic instrument;

[0039] Fig. 1b schematically illustrates an embodiment of a geodetic instrument;

[0040] Fig. 1c schematically illustrates an embodiment of a geodetic instrument;

[0041] Figure 2 Schematically illustrating an image captured by the geodetic instrument;

[0042] Figure 3a And Figure 3b Schematically illustrating processing of the image with a reference image;

[0043] Figure 4a And Figure 4b Schematically illustrating generation of a difference image;

[0044] Figure 5a And Figure 5b Schematically illustrating obtaining of a pixel of an imaging device in the geodetic instrument and a reference image.

[0045] The drawings are not necessarily to scale, and generally only show parts which are necessary in order to illustrate the present inventive concept, wherein other parts can be omitted or merely suggested. DETAILED DESCRIPTION

[0046] The present inventive concept generally relates to the optical functionality of a geodetic instrument, e.g. for surveying. In particular, the operation of a geodetic instrument according to the present inventive concept is advantageous in a geodetic instrument having a specific optical setup as will be further described below with reference to Figs. 1a, 1b and 1c. Such a geodetic instrument will allow easy integration of different functional modules into the geodetic instrument through a simple interface, such that multiple optical paths overlap at least along a lens arrangement. Thus, the optical paths can have the same lenses of the common lens arrangement. In more detail, the separation of the optical means of the lens arrangement from the distance measurement unit, the imaging unit and the tracker provides flexibility in the design of the geodetic instrument, wherein a clear and simple interface allows adding or replacing such functional modules.

[0047] In short, in one embodiment, the geodetic instrument comprises 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 measuring unit (or electronic distance measuring unit, EDM unit) configured to measure a distance to the target along an optical axis of the light source; and a beamsplitter / combiner. The beamsplitter / combiner combines the partial optical paths of the imaging unit and the light source together. Thus, the light source can be part of the distance measuring unit. However, it will be understood that the light pulses emitted by the light source are not necessarily distance measuring light or distance measuring pulses. Although the present disclosure relates to distance measuring light, it should be understood that the present disclosure also includes embodiments in which 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.

[0048] 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 at least between the lens arrangement and the beamsplitter / combiner coaxial with the optical axis of the lens arrangement, i.e. parallel and overlapping.

[0049] Fig. la illustrates elements of a geodetic instrument 100A according to an embodiment. The geodetic instrument 100A comprises a lens arrangement 110, an imaging unit 120, a light source 140 and a beamsplitter / combiner 150.

[0050] The lens arrangement 110 is provided for observing an object, such as a reflector, as a target. The optical arrangement 110 comprises at least one focus lens element 116 arranged moveably for focusing to observe the object. For example, the focus lens element can be a single or compound focus lens that is moved manually or automatically 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 art of surveying and can comprise a plurality of optical lenses, such as lenses 114 and 116, in order to enable focusing and zooming.

[0051] 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 eyepiece (a.k.a. ocular) such that the image can be obtained in the eye of a user. 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 recorded and viewed with the eye through the eyepiece conventionally. Preferably, the imaging unit 120 is an electronic imaging device, e.g. an array of two-dimensional sensor elements capable of generating image information in which the number of pixels generally corresponds to the number of array elements, such as a charge-coupled device (CCD) camera or a complementary metal-oxide-semiconductor (CMOS) camera. Such a sensor array can consist of 1000 x 1000 or more sensor elements to generate an image with 106 A digital image of one image pixel (1 megapixel) or more image pixels. However, smaller sensor arrays are also possible, for example a sensor array consisting of e.g. 480 x 750 sensor elements. Alternatively, the sensor array can consist of avalanche photodiodes (APDs) forming an array of APDs. The light source 140 can be configured to measure a distance to an object along an optical axis of the light source 140 constituting an 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 with respect to the distance. In one example, the light source comprises a distance measurement unit 140 comprising a coherent light source, such as an infrared laser or another suitable laser (which for example emits in the red wavelength range). The distance measurement unit can preferably comprise a fast reflectorless working EDM. Conventionally, collimated light is emitted from the measurement 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 back-reflection signal of the electro-optical distance measurement) can also be used to obtain information with respect to the distance to a reflecting object.

[0052] In Fig. 1a, the 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 arranged coaxially with the optical axis of the lens arrangement at least between the lens arrangement 110 and the beam splitter / combiner 150.

[0053] An optical axis can be seen as an imaginary line defining the path along which light propagates through a 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. An optical path can be seen as the path taken by light as it passes through an optical system, such as the lenses of the lens arrangement 110. An optical path (beam path) is typically confined to a three-dimensional volume with the optical axis as the axis of rotational symmetry in which light can travel.

[0054] The optical axis of the imaging unit 120 is shown by the dash-dotted line and the optical axis of the light source 140 is shown by the dashed line. These lines do not give a preferred direction (optical reciprocity), but it can be considered that light to be imaged on the imaging unit 120 travels from left to right. The beam splitter / combiner 150 combines these lines to make the optical axes overlap on the left side of the beam splitter / combiner 150 in Fig. 1a. Since these axes are parallel to each other and overlap (i.e. coaxial), and in addition parallel to and overlapping with the optical axis of the lens arrangement 110, these axes are also seen as being arranged coaxially with respect to the optical axis of the lens arrangement 110.

[0055] In particular, the optical setup, and especially the beam splitter / combiner 150, is chosen such that the optical axes of the lens arrangement 110 correspond 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 travelling 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 overlap partially, the optical paths, i.e. the optical imaging path and the optical distance measurement path, of these units also overlap partially when combined by the beam splitter / combiner 150. In this arrangement, the center of the two-dimensional sensor element array as imaging unit and the center of the tracking unit do not need to coincide with the optical axes of the lens arrangement 110. For example, in a calibration step, the center can be defined on the two-dimensional sensor element array as the point where the optical axis coincides with the array.

[0056] It will be understood that the coaxial arrangement of the two or more optical axes is basically a theoretical assumption, since in practice the optical axes will not overlap completely and point in exactly the same direction, but will overlap within a certain small error range. Therefore, for ease of explanation, it is assumed that an axis deviation of less than + / - 0.2° is still coaxial. Typical alignment errors are in the order of + / - 0.1°, which can be corrected by software in a later calibration.

[0057] Looking at Fig. 1a from a different angle, according to the principle of optical reciprocity, light coming from the left side of Fig. 1a and thus entering the lens arrangement 110 from the left will be split by the beam splitter / combiner 150. Therefore, describing one optical path being split by the beam splitter / combiner into two optical paths (seen from left to right) and describing two optical paths being combined by the beam splitter / combiner (seen from right to left) is technically the same.

[0058] In a simple case, a semi-transparent mirror can be used as a beam splitter / combiner to split the incoming 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 undesired distance measurement light in the imaging channel can be filtered out before hitting the imaging unit 120. However, in practice a dichroic mirror or a dichroic prism will be used, i.e. a mirror or a prism that is transparent for one range of wavelengths and reflective for another range of wavelengths. This wavelength selectivity can be achieved by using dichroic filters / films using the thin-film interference principle. Thus, using a dichroic mirror or a dichroic prism allows to use a larger percentage of reflected light and transmitted light, respectively.

[0059] Thus, depending on the direction and the wavelength of the light, the beam splitter / combiner 150 is configured to combine the light beams in addition to being configured to split the light passing through the lens arrangement that is reflected from the object into imaging light along the optical imaging path and distance measurement light along the optical distance measurement path.

[0060] From the above, the optical path and optical axis are independent of the direction of light travel, so the splitting and combining are only for better explanation of 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 light from the imaging unit and the light source together, but it is configured with the optical functionality that it can do so, because the light coming into the geodetic instrument through the lens arrangement is split by the beam splitter / combiner into different channels. In other words, the optical functionality of the beam splitter / combiner is to combine different paths coming from its right side to overlap on its left side.

[0061] 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 objects reflecting visible wavelengths. Thus, if a dichroic mirror is provided that has a cut-off wavelength of about 620 nm (i.e., reflects wavelengths larger than 620 nm), then distance measurement and imaging can be achieved in different channels with almost no intensity loss. Using a dichroic prism design further allows to glue the camera chip of the imaging unit directly onto a part of the prism, enabling a highly compact structure that is largely insensitive to temperature changes and external impacts, while at the same time saving mechanical parts for attaching and aligning the camera chip.

[0062] If the lens 116 facing the beam splitter / combiner in the lens arrangement 110 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 measurement device can be achieved. As a result, reflections of the distance measurement light from the light source 140 from this lens will not be reflected back to the distance measurement unit 140, thereby avoiding cross-talk that can lead to detection of signals not coming from the actual target (object). Furthermore, an anti-reflective coating on the lenses of the lens arrangement can also reduce cross-talk. When using a prism as the beam splitter / combiner 150, the intermediate focus point should be placed outside the prism and not inside, and the prism surface(s) on which the light is incident can be slightly tilted with respect to the orthogonal direction, so that the light is not incident on it exactly orthogonally. Furthermore, an air gap between the individual prisms can be provided in place to achieve total reflection.

[0063] In Fig. lb, another embodiment of a geodetic instrument is provided, which is further built on top of the geodetic instrument 100A of Fig. la. In particular, the geodetic instrument 100B comprises the same elements as the geodetic instrument 100A and additionally comprises a tracker 130.

[0064] The tracker 130 is configured to track an object, e.g. a prismatic reflector, by preferably using infrared light with a wavelength of 850 nm (or 810 nm). As directly understood from Fig. lb, the beam splitter / combiner 150 of Fig. la needs some modifications to combine / separate the three beam paths of the tracker 130, the imaging unit 120 and the light source 140, respectively. Thus, in Fig. lb the beam splitter / combiner 150’ is configured to combine a part of the optical tracker 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 axis of the tracker, the optical axis of the imaging unit and the optical axis of the light source are arranged coaxially with the optical axis of the lens arrangement 110 at least between the lens arrangement and the beam splitter / combiner 150’. Thus, the lens arrangement 110 is shared by the tracking, the aiming assistance and / or the distance measurement and imaging functionality.

[0065] In more detail, in Fig. lb 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 dotted-dashed line 121 and the optical axis of the light source 140 is shown by the dashed line 141. 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 is schematically shown in Fig. lb.

[0066] The prism system shown in Fig. lb is a multi-pass prism. In particular, the prism system comprises two prisms having a wedge shape. In a preferred embodiment, the beam splitter / combiner 150’ comprises at least two wedge-shaped 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) can also comprise a wavelength-selective surface. The more light paths that need to be combined, the more prisms or mirrors (or a combination thereof) need to be provided. Thus, in a preferred embodiment with three functional modules, like 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.

[0067] The skilled person realizes that instead of the two dichroic prisms shown in Fig. lb, two dichroic mirrors can also be used. Thus, similar to Fig. la, the optical axes of the tracker, the light source and the imaging unit can be arranged coaxially with the optical axis of the lens arrangement 110 at the left side of the beam splitter / combiner 150’.

[0068] Figure 1CElements of another example of a geodetic instrument 100C are shown. The geodetic instrument 100C comprises a lens arrangement 110, an imaging unit 120, a tracker 130, and a beamsplitter / combiner 150. The geodetic instrument 100C corresponds to the geodetic instrument 100A, but the light source 140 is replaced by the tracker 130, the details of which have been discussed in relation to Fig. lb.

[0069] Figure 2 An image 170 captured by a geodetic instrument is shown, the geodetic instrument comprising a light source for assisting a user in aiming at a target in a scene by emitting light pulses to form a light spot at the target, and an imaging device. The imaging device and the light source share a common light channel within the geodetic instrument, as shown for instance in the geodetic instruments shown in Figs. la-c. The image 170 will be referred to as the “second image” in the following to distinguish it from other images. The second image 170 is captured with the light source on, and can be said to comprise a scene 172, crosstalk 174 occurring in the common light channel, and a light spot 176, preferably generated by light emitted from the light source being illuminated on a target in the scene and being reflected back to the imaging device by the target. It should be noted that, Figure 2 The second image 170, the scene 172, the crosstalk 174, and the light spot 176 are merely schematic representations.

[0070] Reference will now be made to Figure 3a The processing of the second image 170 will be discussed. It should be noted that, Figure 3a The second image 170, the scene 172, the crosstalk 174, and the light spot 176 are merely schematic representations. Figure 3a In the figure, the first row shows the second image 170 and its constituents, the second row shows the reference image 160 and its constituents, and the third row shows a processed image 180 and its constituents, resulting from processing the second image 170 with the reference image 160.

[0071] The second image 170 comprises the scene 172, the crosstalk 174, and preferably the light spot 176. A first image of the scene has been captured by the imaging device with the light source on, which scene is not necessarily the same as the scene in the second image 170. The reference image 160 is obtained from at least the first image, wherein the influence of the scene from the first image is suppressed. Furthermore, in the reference image 160, the influence from the light spot has been suppressed. Thus, the reference image 160 can comprise only the influence from the crosstalk 164 occurring in the common light channel. The crosstalk 164 of the reference image 160 can be substantially the same as the crosstalk 174 present in the second image 170.

[0072] By processing the second image 170 with the reference image 160, the cross-talk 174 in the second image 170 can be removed. Thus, a processed image 180 can be achieved. Since the influence from the light points and the scene in the reference image 160 has been suppressed, the reference image 160 only comprises the cross-talk 164, and the scene 172 and the light points 174 in the second image 170 will remain in the processed image 180. In other words, processing the second image 170 with the reference image 160 will remove or reduce the influence from only the cross-talk 174.

[0073] Reference is now made to Figure 3b , similar to what is discussed in connection with Figure 3a , the second image 170 can be processed. It should be noted that various features of the disclosure are schematically represented in Figure 3b . In Figure 3b , the first row demonstrates the second image 170 and its constituents, the second row demonstrates the reference image 160 and its constituents, and the third row demonstrates the processed image 180 and its constituents, which is produced by processing the second image 170 with the reference image 160.

[0074] The second image 170 comprises the scene 172, the cross-talk 174, and preferably the light points 176. A first image of the scene has been captured by the imaging device with the light source turned on, which is not necessarily the same scene as in the second image 170. The reference image 160 is obtained from at least the first image, wherein the influence from the scene is suppressed. Thus, the reference image 160 can only comprise the influence from the cross-talk 164 occurring in the common light channel and from the light points 166. The influence from the cross-talk 164 in the reference image 160 can be substantially the same as the influence from the cross-talk 174 in the second image 170. Similarly, the influence from the light points 166 in the reference image 160 can be substantially the same as the influence from the light points 174 in the second image 170.

[0075] By processing the second image 170 with the reference image 160, the cross-talk 174 in the second image 170 can be removed. Thus, a processed image 180 can be achieved. Since the influence from the light points and the scene in the reference image 160 has been suppressed, the reference image 160 only comprises the cross-talk 164, and the scene 172 and the light points 174 in the second image 170 will remain in the processed image 180. In other words, processing the second image 170 with the reference image 160 will remove or reduce the influence from only the cross-talk 174.

[0076] However, it has been recognised that the effect from the light spot in the image will depend on the distance from the light source to the target that is illuminated by the light emitted by the light source. Thus, given that the reference image and the second image are captured at different arrangements of the target, the effect from the light spot in the reference image and the second image can be different. Thus, the reference image 160 preferably only includes the effect from the cross-talk 164, as described in relation to Figure 3a

[0077] The process of generating the difference image will now be described with reference to Figure 4a It should be noted that Figure 4a are merely illustrative representations of various features disclosed. In Figure 4a the first row illustrates the first image 190 and its components, the second row illustrates the third image 200 and its components, and the third row illustrates the difference image 210 and its components, which is generated based on the first image 190 and the third image 200.

[0078] The first image 190 includes a scene 192 and cross-talk 194. Such a first image 190 can be captured, for example, by suppressing the effect from the light spot, for example by aiming the geodetic instrument such that the light source is directed away from reflecting light emitted by the light source back to the imaging device when capturing the first image 190.

[0079] The third image 200 of the scene, which is captured with the light source turned off, only includes the scene 202. Thus, a difference image 210 can be generated based on the first image 190 and the third image 200, which only includes the effect from the cross-talk 194. The reference image can then be based on the difference image 210, and the reference image can be used as described in relation to Figure 3a In other words, the difference image 210 can be the reference image.

[0080] As will be understood from the present disclosure, the scene 192 in the first image 190 and the scene 202 in the third image 200 are preferably the same scene aligned and / or captured under the same settings with respect to ambient light, such that the difference image of the first image 190 and the third image 200 will have the effect from the scenes 192, 202 completely erased in the difference image. Furthermore, the scene of the first image 190 and the scene of the third image 200 being denoted with different reference numerals in the present disclosure should not be understood to indicate that these scenes are necessarily different from each other. Preferably, the scene of the first image 190 is the same or substantially the same as the scene of the third image 200. In some cases, the scene of the first image 190 and the scene of the third image 200 can be slightly different at least in certain parts. For example, due to temporal aspects between the capturing of the first image 190 and the capturing of the third image 200, there can be some changes or disturbances, such as changes in ambient light or minor scene changes. ​

[0081] Reference will now be made to Figure 4b The process of generating a difference image will be described. It should be noted that Figure 4b are merely illustrative representations of the various features disclosed. In Figure 4b , the first row illustrates a first image 190 and its components, the second row illustrates a third image 200 and its components, and the third row illustrates a difference image 210 and its components, which is generated based on the first image 190 and the third image 200.

[0082] The first image 190 includes a scene 192, crosstalk 194, and a light point 196. The third image 200 of the scene, which is captured with the light source turned off, includes only the scene 202. Thus, based on (or using) the first image 190 and the third image 200, a difference image 210 can be generated by subtracting the second image 200 from the first image 190, and the difference image 210 includes only the influence from the crosstalk 194 and the light point 196. The reference image can then be based on the difference image 210, and the reference image can be used as described in connection with Figure 3b . In other words, the difference image 210 can be the reference image.

[0083] As will be understood from the present disclosure, the scene 192 in the first image 190 and the scene 202 in the third image 200 are preferably the same scene aligned and / or captured under the same settings with respect to ambient light, such that the difference image of the first image 190 and the third image 200 will have the influence from the scenes 192, 202 completely erased in the difference image. Moreover, the scenes of the first image 190 and the scenes of the third image 200 being denoted with different reference numerals in the present disclosure should not be understood to indicate that these scenes are necessarily different from each other.

[0084] Reference is now made to Figure 5a and Figure 5b , which schematically illustrate pixels of an imaging device in a geodetic instrument. The pixels can record an intensity level related to the intensity of light impinging at the pixel. For the sake of clarity, in the schematic illustration, the pixels can record a low intensity level (depicted in the figure with black pixels), a medium intensity level (depicted in the figure with grey pixels), or a high intensity level (depicted in the figure with white pixels). It should be understood that the low intensity level can correspond to a zero intensity level, i.e., no light is recorded. Moreover, as will be readily understood from the present disclosure, the inventive concept described herein can be applied to imaging devices having pixels with any number of possible recorded intensity levels.

[0085] In some embodiments, a method for operating a geodetic instrument includes capturing a plurality of images with a light source turned on. Reference Figure 5aThe first image 222, the second image 224 and the third image 226 of the plurality of images are illustrated in FIG. 2. Each of the images 222, 224, 226 comprises a plurality of pixels corresponding to the pixels of the imaging device.

[0086] The first image 222 comprises a plurality of pixels 220. In the first image 222, some of the plurality of pixels 220 have recorded a low intensity level and are thus low intensity level pixels 228, other pixels have recorded a medium intensity level and are thus medium intensity level pixels 230, and yet other pixels have recorded a high intensity level and are thus high intensity level pixels 232.

[0087] Likewise, the second image 224 comprises a plurality of pixels 220. The second image 224 also comprises low intensity level pixels 228, medium intensity level pixels 230 and high intensity level pixels 232. However, depending on various conditions, such as the aiming target of the geodetic instrument, the scene, the target to be observed in the scene, the ambient light, etc., the pixels of the plurality of pixels 220 can record different intensity levels for the second image 224 than for the first image 222. Similarly, the third image 226 comprises a plurality of pixels 220, some of which can record different intensity levels than the first image 222 and / or the second image 224.

[0088] The lowest recorded intensity level of each pixel of the imaging device in the plurality of images (i.e. the first image 222, the second image 224 and the third image 226) can be determined. The reference image can then be obtained using the lowest recorded intensity level determined for each respective pixel of the imaging device. Figure 5b The reference image 234 using the lowest recorded intensity level determined for each respective pixel of the imaging device in the first image 222, the second image 224 and the third image 226 is illustrated in FIG. 2. It can be seen that the lowest recorded intensity level of each pixel (i.e. the reference image) here corresponds to the crosstalk occurring in the common light channel. Such a reference image 234 can be used to process images captured with the light source on to remove the crosstalk from the images.

[0089] A method for operating a geodetic instrument according to the inventive concept will now be described with reference to Figure 6a and Figure 6b It is emphasized that some method features are not necessarily processes delimited in time or separate from each other, and more than one "process feature" can be executed simultaneously in a parallel manner.

[0090] A method includes capturing 650 a first image of a scene with a light source on; obtaining 652 a reference image from at least the first image, wherein an influence from the scene is suppressed, the reference image representing crosstalk occurring in a common light channel; capturing 654 a second image with the light source on; and processing 656 the second image with the reference image to remove crosstalk from the second image.

[0091] A method includes capturing 670 a plurality of images with a light source on; determining 672 a lowest recorded intensity level for each pixel of an imaging device in the plurality of images; obtaining 674 a reference image using the lowest recorded intensity level determined for each respective pixel of the imaging device, the reference image representing crosstalk occurring in a common light channel; capturing 676 an image to be processed with the light source on; and processing 678 the captured image to be processed with the reference image to remove crosstalk in the image to be processed.

[0092] As those skilled in the art will readily appreciate from the disclosure of the principles of the present concept herein, numerous modifications and variations can be made thereof. Such modifications and variations are intended to be included within the scope of the present concept as defined by the above-mentioned patent claims.

Claims

1. 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 by emitting a light pulse to form a light spot at the target, and an imaging device, wherein, The imaging device and the light source share a common light channel within the geodetic instrument, the method comprising: capturing a first image of the scene with the light source turned on; obtaining a reference image from at least the first image, wherein an influence from the scene is suppressed, the reference image representing crosstalk occurring in the common light channel; capturing a second image with the light source turned on; and processing the second image with the reference image to remove crosstalk from the second image.

2. The method of claim 1, wherein, The influence from the light spot in the reference image is suppressed by aiming the geodetic instrument such that the light source is directed at a target that does not reflect light emitted by the light source back to the imaging device when the first image is captured.

3. The method of claim 1 or 2, wherein, The influence from the scene is suppressed by providing a dark scene or a scene that does not reflect ambient light towards the imaging device.

4. The method according to claim 1 or 2, further comprising: capturing a third image of the scene with the light source turned off; and generating a difference image based on the first image and the third image, wherein an influence from the scene is suppressed; wherein the reference image is based on the difference image.

5. The method according to claim 1 or 2, further comprising capturing additional images with the light source turned on, the additional images and the first image forming a plurality of images; wherein obtaining the reference image comprises: determining a lowest recorded intensity level for each pixel of the imaging device in the plurality of images; and using the lowest recorded intensity level determined for each respective pixel of the imaging device to obtain the reference image.

6. 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 by emitting a light pulse to form a light spot at the target, and an imaging device, wherein, The imaging device and the light source share a common light channel within the geodetic instrument, the method comprising: capturing a plurality of images with the light source turned on; determining a lowest recorded intensity level for each pixel of the imaging device in the plurality of images; using the lowest recorded intensity level determined for each respective pixel of the imaging device to obtain a reference image, the reference image representing crosstalk occurring in the common light channel; capturing an image to be processed with the light source turned on; and processing the captured image to be processed with the reference image to remove crosstalk in the image to be processed.

7. A geodetic instrument, comprising: a light source for assisting a user in aiming a target in a scene by emitting a pulse of light to form a light spot at the target; an imaging device configured to capture images of the scene using a sequence of frames; wherein the imaging device and the light source share a common light channel within the geodetic instrument; and a processing unit configured to operate the geodetic instrument according to the method as defined in any one of claims 1 to 6.

8. The geodetic instrument of claim 7, wherein, The light source is an infrared laser, and wherein the imaging device is an infrared camera.

9. The geodetic instrument of claim 7, wherein, The light source is a laser in the visible spectrum, and wherein the imaging device is configured to detect visible light.

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