A computer-implemented method for determining the relative position between a first camera and a second camera used in a medical application, wherein "position" means a combination of an orientation given in up to three rotational dimensions and a location given in up to three translational dimensions, the first camera captures a 2D image of a phantom, the second camera emits light onto the phantom and analyzes the reflected light, thus generating a 3D point cloud representing points on the surface of the phantom, and the phantom has a planar surface forming a background which carries a plurality of 2D markers, wherein one of the background and the 2D markers is reflective, thus reflecting light emitted by the second camera back to the second camera, and the other one is non-reflective, thus not reflecting light emitted by the second camera back to the second camera, the method involving that a) the first camera captures a 2D image of the phantom, b) the second camera generates a 3D point cloud representing the planar surface of the phantom, c) the 2D markers are identified in the 2D image, thus obtaining 2D marker data representing the locations of the 2D markers in the 2D image, d) the 2D markers are identified in the 3D point cloud using the property that points on a non-reflective part of the planar surface are identified as having a larger distance to the second camera than points on a reflective part of the planar surface, thus obtaining 3D marker data representing the locations of the 2D markers in a reference system of the second camera, and e) finding the relative position between the first camera and the second camera by applying a Perspective-n-Points algorithm on the 2D marker data and the 3D marker data, wherein step d) involves the steps of - rotating the 3D point cloud such that the planar surface of the phantom is perpendicular to the depth direction of the reference system of the second camera, thus obtaining a 2D view of the 3D point cloud, - finding the 2D markers in the 2D view, thus obtaining auxiliary 2D marker data and - performing an inverse rotation of the auxiliary 2D marker data, thus obtaining the 3D marker data, wherein a plane is fitted into the points of the 3D point cloud which represent the planar surface of the phantom, a transformation describing rotations about axes perpendicular to the depth direction of the reference system of the second camera and making the plane perpendicular to said depth direction is calculated, the transformation is applied to the 3D point cloud and the inverse of the transformation is applied to the auxiliary 2D marker data. a) the first camera captures a 2D image of the phantom, b) the second camera generates a 3D point cloud representing the planar surface of the phantom, c) the 2D markers are identified in the 2D image, thus obtaining 2D marker data representing the locations of the 2D markers in the 2D image, d) the 2D markers are identified in the 3D point cloud using the property that points on a non-reflective part of the planar surface are identified as having a larger distance to the second camera than points on a reflective part of the planar surface, thus obtaining 3D marker data representing the locations of the 2D markers in a reference system of the second camera, and e) finding the relative position between the first camera and the second camera by applying a Perspective-n-Points algorithm on the 2D marker data and the 3D marker data, wherein step d) involves the steps of - rotating the 3D point cloud such that the planar surface of the phantom is perpendicular to the depth direction of the reference system of the second camera, thus obtaining a 2D view of the 3D point cloud, - finding the 2D markers in the 2D view, thus obtaining auxiliary 2D marker data and - performing an inverse rotation of the auxiliary 2D marker data, thus obtaining the 3D marker data, wherein a plane is fitted into the points of the 3D point cloud which represent the planar surface of the phantom, a transformation describing rotations about axes perpendicular to the depth direction of the reference system of the second camera and making the plane perpendicular to said depth direction is calculated, the transformation is applied to the 3D point cloud and the inverse of the transformation is applied to the auxiliary 2D marker data. - rotating the 3D point cloud such that the planar surface of the phantom is perpendicular to the depth direction of the reference system of the second camera, thus obtaining a 2D view of the 3D point cloud, - finding the 2D markers in the 2D view, thus obtaining auxiliary 2D marker data and - performing an inverse rotation of the auxiliary 2D marker data, thus obtaining the 3D marker data, wherein a plane is fitted into the points of the 3D point cloud which represent the planar surface of the phantom, a transformation describing rotations about axes perpendicular to the depth direction of the reference system of the second camera and making the plane perpendicular to said depth direction is calculated, the transformation is applied to the 3D point cloud and the inverse of the transformation is applied to the auxiliary 2D marker data.