Tracker, surveying apparatus and method for tracking a target

1. Tracker for tracking a target, comprising:

an image sensor arrangement having an imaging region composed of a plurality of pixels arranged in a matrix of columns and rows; the imaging region being arranged to take an image of a scene including the target;

a controller configured to:

receive or generate a timing signal, the timing signal indicating a time duration during which an illumination unit illuminating the target in the scene is switched on and off,

control the imaging region to take an image of the scene when the timing signal indicates that the illumination unit is switched on, and read out a subgroup of neighboring columns and rows constituting a stripe window of the imaging region so that an image section including the target is obtained, the subgroup of neighboring columns and rows having an unequal number of columns and rows,

control the imaging region to take another image of the scene when the timing signal indicates that the illumination unit is switched off, and read out another subgroup of neighboring columns and rows constituting another stripe window of the imaging region so that another image section including the target is obtained, the another subgroup of neighboring columns and rows having an unequal number of columns and rows,

calculate a difference image section by determining a difference between the pixel values of the pixels of the image section and the other image section, and

an output unit configured to output the difference image section so as to identify the target in the difference image section.

2. The tracker according to claim 1 , wherein the imaging region of the image sensor arrangement is adapted so that more pixels are arranged in the direction of the rows than in the directions of the columns and is arranged in the tracker so that the direction of the rows corresponds to a vertical direction.

3. The tracker according to claim 1 , wherein the number of columns and rows of a stripe window is between one column or row to half of all columns or rows, and the number of columns and rows of the stripe window is between a twentieth to a third of all columns or rows of the imaging region.

4. The tracker according to claim 1 , wherein in a target search mode a stripe window of a first size is used and in a target tracking mode a stripe window of a second size is used, the first size and the second size being different.

5. The tracker according to claim 1 , wherein the controller is further configured to determine the position of the target inside the stripe window, and if the determined position of the target does not coincide with a position on the center line of the stripe window, the stripe window is enlarged and/or shifted in the imaging region and/or the image sensor arrangement is mechanically moved so that the target is positioned closer to the center line.

6. The tracker according to claim 1 , wherein the controller is configured, after reading out the subgroup and the other subgroup of neighboring columns and rows constituting two stripe windows, when calculating the difference image section, to correct for a movement of the tracker in the time between taking the image and the other image by taking into account an offset between the scene on the stripe window of the image and the scene on the stripe window of the other image, the offset corresponding to a shift in the scene due to the movement of the tracker.

7. The tracker according to claim 6 , wherein to correct for the movement of the tracker, the controller is configured to identify an area of pixels of the stripe window and another area of pixels of the other stripe window which correspond to the same scene so as to obtain a difference area by determining a difference between the pixel values of the pixels of the area and the other area which recorded the same scene.

8. The tracker according to claim 1 , wherein the tracker comprises a tracker emitter for emitting tracking light on an optical tracker path.

9. The tracker according to claim 1 , wherein the tracker is adapted to issue an instruction to a surveying apparatus to move an optical axis of a lens arrangement of the surveying apparatus.

10. Surveying apparatus for surveying a target, comprising

a lens arrangement including at least one movably arranged focus lens element for focusing to sight a target;

an imaging unit configured to obtain an image of at least a part of the target; and

the tracker according to claim 1 .

11. The surveying apparatus according to claim 10 , further comprising

a beam splitter/combiner configured to combine a part of an optical imaging path of the imaging unit and a part of an optical tracker path of the tracker so that an optical axis of the imaging unit and an optical axis of the tracker are at least coaxially arranged with an optical axis of the lens arrangement between the lens arrangement and the beam splitter/combiner.

12. The surveying apparatus according to claim 10 , further comprising

a distance measuring unit configured to measure a distance to the target along an optical axis of the distance measuring unit.

13. The surveying apparatus according to claim 12 , wherein the beam splitter/combiner is further configured to combine the part of the optical tracker path of the tracker, the part of the optical imaging path of the imaging unit and a part of an optical distance measuring path of the distance measuring unit so that the optical axis of the tracker, the optical axis of the imaging unit and the optical axis of the distance measuring unit are at least coaxially arranged with the optical axis of the lens arrangement between the lens arrangement and the beam splitter/combiner.

14. Method for tracking a target, comprising:

taking an image of a scene including the target on an imaging region of an image sensor arrangement, which is composed of a plurality of pixels arranged in a matrix of columns and rows; and

taking another image of a scene including the target on the imaging region;

wherein when receiving or generating a timing signal indicating a time duration during which an illumination unit illuminating the target in the scene is switched on and off,

the imaging region is controlled to take the image of the scene when the timing signal indicates that the illumination unit is switched on, and a subgroup of neighboring columns and rows constituting a stripe window of the imaging region is read out so that an image section including the target is obtained, the subgroup of neighboring columns and rows having an unequal number of columns and rows, and

the imaging region is further controlled to take another image of the scene when the timing signal indicates that the illumination unit is switched off, and another subgroup of neighboring columns and rows constituting another stripe window of the imaging region is read out so that another image section including the target is obtained, the another subgroup of neighboring columns and rows having an unequal number columns and rows, and the method further comprising:

calculating a difference image section by determining a difference between the pixel values of the pixels of the image section and the other image section, and

outputting the difference image section so as to identify the target in the difference image section.

15. A program including instructions adapted to cause data processing means to carry out the method of claim 14 .